| Simple really, it has a high content of hydrogen chloride (hydrochloric acid), which the freezing point as around -114degC.
Your battery acid will freeze well above this point though, because there's a high water content which , as we all know, freezes at 0degC.
So depending on the percentages of HCl and H2O that you have in your battery, the mixture will freeze at somewhere between -114 and 0. | 
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